The present disclosure relates to subject matter contained in priority Japanese Patent Application No. 2001-140964, filed on May 11, 2001, the contents of which is herein expressly incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an alkaline rechargeable battery, and more particularly to an alkaline rechargeable battery having an extended life by improving separators.
2. Description of Related Art
As an example of an alkaline rechargeable battery, a nickel-metal hydride rechargeable battery including positive electrode plates having nickel hydroxide as its active material, negative electrode plates having hydrogen-absorption alloy, and separators interposed therebetween, has been known and widely used.
With the alkaline rechargeable battery of this type, in order to increase a battery capacity, the separators are made as thin as possible to the extent that they are able to absorb a necessary quantity of electrolyte while functioning properly to prevent a short-circuit. In general, a volume of the separators is set to less than 30% of a volume of the electrode plate group.
However, the electrode plates gradually expand while the battery is charged and discharged repetitively, and when the volume of the separators is set to less than 30% of the volume of the electrode plate group, the expanded electrode plates crush the separators, and a quantity of the electrolyte retained in the separators is reduced. As a result, an internal resistance of the battery increases, which causes heat generation. Accordingly, an internal pressure increases and the safety vent operates to release a generated gas to the outside of the battery, whereby the electrolyte dries up and the battery becomes unusable in a short period.
In light of the conventional problems as discussed above, an object of the present invention is to provide an alkaline rechargeable battery having an extended life by improving separators.
An alkaline rechargeable battery according to the present invention includes an electrode plate group including positive electrode plates and negative electrode plates being superimposed with separators interposed therebetween, wherein a volume of the separators at least before initial charge and discharge (a period from assemblage to an initial charge of the battery) accounts for 30 to 60% of a volume of the electrode plate group. By setting a proportion of the separators in the electrode plate group in volume to 30% or higher, compression of the separators that shortens the service life of the battery is prevented. When the proportion is raised to more than 60%, it becomes difficult to release heat generated inside the battery to the outside, which deteriorates the life characteristics. Hence, by setting the proportion to 30 to 60%, a total quantity of electricity that the battery can be charged and discharged is increased and the life characteristics are enhanced, thereby providing a battery maintaining a high reliability over a long period.
In addition, it is preferable to set a plurality of voids of the separators before the initial charge and discharge to account for 50 to 70% of the separators. When a proportion of the voids exceeds 70%, a short-circuit readily occurs. When the proportion of the voids is lower than 50%, a volume for retaining an electrolyte is reduced, and the life characteristics are deteriorated.
Further, it is preferable to set a percentage of the volume of the electrode plate group before the initial charge and discharge in a capacity of a battery case to 85 to 95%. When the percentage is lower than 85%, an interval between the electrode plates becomes so large that the output characteristics are deteriorated, which increases an internal resistance and the life characteristics are deteriorated. When the percentage exceeds 95%, the separators are compressed, which makes it difficult for the separators to retain an electrolyte, and the life characteristics are deteriorated.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.